The present invention relates to an improved system for compensating for thermal expansion for use with a measuring device of the type which includes a measuring scale connected to a first object, and a scanning unit connected to a second object and positioned to scan the scale to determine the position of the second object with respect to the first object.
Arrangements in measuring systems for machine tools for compensating for thermally-induced length changes of machine tool parts are known, for example, from German Patent DE-PS No. 31 06 701. The system disclosed in this patent includes an expansion bar formed of a material having a high coefficient of thermal expansion. This expansion bar is fastened at one end to a machine part and at the other end to a measuring scale. Thermal expansion of the expansion bar is equal to that of the machine tool part such that thermal expansion of the machine part is compensated for in the measuring system.
In this arrangement, the effective length of the expansion bar, as measured between the fastening points of the expansion bar to the machine part and to the scale, is determined by the magnitude of the thermally-induced length changes of the machine part. This preset effective length of the expansion bar is specific for the particular machine part for which is was made, and therefor the measuring system cannot be used with machine parts having other temperature characteristics without alteration to the measuring system.